1. Field of the Invention
The present invention generally relates to a circuit and method for reducing fixed pattern noise. In particular, the present invention relates to a circuit and method for reducing fixed pattern noise in a CMOS image sensor.
2. Description of the Related Art
FIG. 1 illustrates a conventional CMOS image sensor 100 using a Bayer-type color filter array as shown in U.S. Pat. No. 3,971,065. A color pixel array using the Bayer type color filter array has three types of pixels, red 120, blue 150 and green 130, 140. GR 130 is a green pixel located in the same row as a red pixel 120, and GB 140 is a green pixel located in same row as a blue pixel 150.
Each color image signal from each pixel 120, 130, 140, 150 is converted to a digital signal by an analog to digital converter (ADC) 180. An ADC 180 is connected to each column of pixels. Each column has one ADC 180.
Image signals from a single row (each row includes pixels from plural columns) is selected by a row driver 110, held and sampled by a correlated double sampling block (CDS) 160, amplified by an amplifier 170 and converted to digital signals by the ADC 180 of each column. The digital signals from the ADC 180 are latched in a latch block 190 and outputted in series by a data selector 195, in accordance with a selection signal.
As illustrated in FIG. 1, each of the ADCs 180 are on the same side of the color filter array. Because the width of the CDS (or ADC) must be same to the pitch of the unit pixel, it is difficulty to lay out a high performance CDS(or ADC) in one column pitch.
Another design which permits high performance ADC within one column pitch is illustrated in FIG. 2. In FIG. 2, odd/even column circuits are split between opposite sides of the color filter array, for example, the top and bottom of the color filter array.
Operation of the CMOS image sensor 200 in FIG. 2 is similar to that of the CMOS image sensor 100 of FIG. 1. Each electric signal from a red pixel 235, GR pixel 240, GB pixel 245 and blue pixel 250 is input to one of column ADC blocks 207, 208. As shown in FIG. 2, the signals from each pixel in an odd column of pixels are processed in a lower column ADC block 208 which is located under the pixel array. The ADC block 208 includes CDS blocks 232, amplifiers 227, ADCs 222, a latch block 217, and a data selector 210. The signals from each pixel in an even column of pixels are processed in the upper column ADC block 207 above the pixel array. The ADC block 207 also includes CDS blocks 230, amplifiers 225, ADCs 220, a latch block 215, and a data selector 212.
After a digital signal from the ADC 220 or 222 is latched in the latch block 215 or 217, the data selector 210 or 212 outputs the data to a multiplexer (MUX) 260. The MUX 260 arranges both data from the upper and lower column ADC blocks 207, 208 in series and outputs the serialized data to an image signal processor (not shown).
Due to the spatial difference between the upper column ADC block 207 and the lower column ADC block 208, fixed pattern noise (FPN) is generated. FPN, also called non-uniformity, is a spatial variation, due to device and interconnect parameter variations across the CMOS image sensor 200. FIG. 3 is a graph of which shows a difference in light sensitivity between GR and GB pixels, compared to an increasing quantity of received light intensity.
The slope of each curve indicates a gain of each color. The red and blue color pixels each have one slope. However, the green color has two slopes, one for the GR pixels processed by ADC block 207 and one for the GB pixels processed by ADC block 208.
FIG. 4A illustrates a conventional demosaic method for both the red 235 and blue 250 pixels. As shown in the illustration of FIG. 4A and the associated calculation, for the red 235 and blue 250 pixels in the color filter array, the difference between the green color gains GR and GB does not degrade the image. However, FIG. 4B illustrates a conventional demosaic method for both the GR 240 and GB 245 pixels. As shown in the illustration of FIG. 4B and the associated calculation, for the GR 240 and GB 245 pixels in the color filter array, the difference between the green color gains GR and GB does degrade the image. This difference is manifested as a “checker pattern” in the image and can result in significant image quality degradation. This undesirable “checker pattern” is illustrated in FIGS. 5A and 5B.